Given McKay's affinity for tech etc on other worlds it would seem like Mad Science would be the most appropriate, however I think that in the Weird West, perhaps because things are weird, his powers are blessed.

I think he will act as if they are a curse, perhaps some devil came to him as he lay gut shot somewhere and offered to save him and give him the power to enact his revenge for some future debt yet to be announced. Besides how much of a blessing can it be to see the Flaw in All Things, always seeing what is wrong, where things are broken etc